The present invention relates to nuclear reactor fuel element assemblies and, more particularly, to a type of fuel assembly characterized by elongated cylindrical fuel rods supported in a bundle, the fuel rods being substantially parallel to each other. Such a fuel assembly typically employs spacer grids located at regular intervals along its length. These spacer grids are typically made from slotted thin metal strips interlocked in an egg crate arrangement and joined at their intersections by welds. The spacer grids help to locate and support the fuel rods. Specifically, the fuel rods are passed axially through the square openings formed by the interlocking metal strips of the spacer grid, and are positioned by hard (i.e. inflexible) and soft (i.e. flexible) stops formed from the side walls in each grid cell.
In present practice, control rod guide tubes and an instrument guide tube are interspersed in a regular pattern through the fuel rod array. The guide tubes are unfueled elongated cylinders similar to the fuel rods.
In a typical arrangement, a 15.times.15 array of fuel rods is employed, in which the center position in the array is reserved for a central instrument guide tube. Another sixteen positions are provided with guide tubes for use as control rod locations.
The control rod guide tubes are open at their upper ends to permit passage of control rods into the fuel assembly for the purpose of regulating the fission reaction. Similarly, the instrument guide tube is open at its lower end to permit passage of an instrument probe into the fuel assembly for the purpose of monitoring the neutronic and thermal conditions in the fuel assembly during operation. Typically, the guide tubes are rigidly attached by threaded mechanical joints or welds at each end to upper and lower end fittings.
These end fittings consist of a grillage with openings to accommodate the guide tube ends and to permit the passage of coolant flow through the fuel assembly. Other structural features are typically attached to these grillages to facilitate the positioning of the fuel assembly in the reactor core and to interface with handling devices. One or more helical springs are mounted on the upper end fitting to prevent fuel assembly lift-off caused by the upward flow of coolant through the fuel assembly.
In many current fuel assembly designs, the spacer grids discussed above are rigidly fixed to the guide tubes either by welding or by mechanical attachment. At variance with this rigid design is the floating grid arrangement shown in the Babcock & Wilcox fuel assembly design, in which no mechanical attachment or weldment is provided. Instead, the spacer grids are left free to slip axially along the guide tubes to accommodate minor changes in the axial length of the fuel rods during irradiation.
Spacer sleeves located around the central instrument guide tube are designed to prevent excessive axial relocation of the spacer grids during irradiation. These sleeves are typically located axially between the spacer grids, i.e. in the axial intervals between spacer grids, and are sized to be shorter than the distance between adjacent spacer grids, leaving a gap for axial relocation of the spacer grids.
The spacer grids of the Babcock & Wilcox design have found to "float" upward on the fuel rod bundle during irradiation. This occurs during the third cycle with grids made of Inconel, and during the first cycle with Zircaloy grids. The Zircaloy grids also move more once they begin to slip, going to the limits provided by the spacer sleeves rather than moving only a fraction of an inch as occurs with the use of Inconel as a grid material.
The central instrument tube is positioned in the spacer grid by saddles which are formed from the spacer grid strips. These saddles are dimples or projections which protrude from the top and bottom edge of the central spacer grid strips. These saddles are relatively weak and can be turned inside out by the spacer sleeves if sufficient upward force is exerted by the grid. The Zircaloy grid saddles are only about one-half as strong as the Inconel grid saddles.
The spacer sleeves bear on the top and bottom edges of the upper and lower saddles respectively at different times in an operating cycle. If a high axial load is applied to the spacer sleeves by the grid, the saddles may fail as cantilever beams turning inward and damaging the grid. A relative displacement of the spacer grid with respect to adjacent fuel assemblies may occur, causing possible fretting of fuel rods and a more uneven reactor core geometry. A need thereof arises to find a means for limiting spacer grid axial displacement more than is possible presently, in order to insure grid overlap between adjacent fuel assemblies.
Several possible arrangements have been investigated to resolve this problem. Direct attachment means, such as welding the spacer grids to the guide tubes, is undesirable for two reasons: the manufacturing sequence is severely disrupted, and the basic advantages associated with a floating grid arrangement are lost. Swaging the central instrument guide tubes above and below the grids in order to limit travel is a feasible alternative, but also significantly alters the fabrication sequence.
The best approach is to retain the floating grid concept, lengthen the spacer sleeve to achieve the grid positioning improvement required, and strengthen the central instrument guide tube saddles, (especially on the Zircaloy spacer grid) to carry the resulting loads.